The present invention relates to solar cells, particularly to high efficiency solar cells and method of fabricating same, and more particularly to high efficiency solar cells which use deep p and n contacts formed by pulsed laser doping to create high electric fields within the carrier collection volume material of the cell.
Solar cells are being widely used in a variety of different applications. In space applications, for example, there is a need for light weight, low cost, but high efficiency solar cells.
Over the years, there has been a substantial research and development effort to produce more efficient solar cells without increased cost or weight, particularly for space applications. The following exemplify the more recent efforts.
Wakefield et al., U.S. Pat. No. 4,602,120 issued Jul. 22, 1986 discloses a solar cell wherein a backside reverse p-n junction is formed prior to metalization and a gridded backside metalization material, chosen so that the material penetrates the p-n junction where present, is fired into the wafer.
Basol et al., U.S. Pat. No. 4,675,468 issued Jun. 23, 1987 discloses a thin-film photo voltaic device including a stable front contact current collector grid.
Mardesich, U.S. Pat. No. 4,703,553 issued Nov. 3, 1987 discloses a drive through doping process which involves substrate heating for manufacturing low back surface recombination solar cells. Deep impurity dotted regions are provided under the back contacts of the cell using a process wherein a metal paste drives through an oxide layer and alloy with selected regions of a p+ type layer and a p- type layer to a predetermined depth thereby forming heavily doped p+ regions.
Wenham et al., U.S. Pat. No. 4,726,850 issued Feb. 23, 1988 discloses a buried contact solar cell wherein an electrical contact is formed in a groove in the surface of the semiconductor material.
Barnett et al., U.S. Pat. No. 4,781,766 issued Nov. 1, 1988 discloses a thin-film solar cell made up of semiconductor layers formed on an aluminum silicon eutectic alloy substrate.
Cogan et al., U.S. Pat. No. 4,824,489 issued Apr. 25, 1989 discloses a method for processing solar cells which involves thinning silicon substrates and a transparent substrate, such as glass, is secured to the silicon by a low temperature adhesive prior to the thinning operation to prevent breaking of the thinned silicon substrate. An amorphous doped semiconductor material is applied to the back surface of the thinned silicon substrate and exposed to pulsed laser energy for melting the amorphous material without causing thermal damage to the adhesive material bonding the thinned silicon substrate to the glass substrate.
Swanson et al., U.S. Pat. No. 5,030,295 issued Jul. 9, 1991 discloses a silicon solar cell wherein the interface of a silicon oxide passivation layer and a silicon substrate in a silicon solar cell is stabilized by a layer of undoped or phosphorous doped polycrystalline silicon.
Sinton, U.S. Pat. No. 5,053,083 issued Oct. 1, 1991 discloses a bi-level contact solar cell wherein the back surface of the cell is patterned by etching into an array of bi-level, interdigitated mesas and trenches, separated by inclined surfaces.
While these prior efforts have served to advance the state of solar cells and fabrication techniques therefor, there is a need for higher efficiency solar cells. Such a need is provided by the present invention wherein the solar cell uses deep contacts which penetrate the electric field deep into the material where the high strength of the field can collect many of the carriers. This is especially beneficial for materials such as polycrystalline, microcrystalline, and amorphous silicon, which all have limited carrier lifetimes relative to crystalline silicon (c-Si), due to recombination sites at grain boundaries and open bond sites. The deep contacts are provided by repetitive pulsed laser doping.